This invention relates to tensiometers for measuring soil water potential and more specifically to a tensiometer that self-compensates to changing water levels and pressures in the tensiometer reservoir, thereby permitting longer useful operating periods.
The use of tensiometers for monitoring moisture potential in soil is known. For example, our invention claimed in U.S. Pat. Nos. 5,644,947 and 5,915,476 (which are hereby incorporated by reference) describe portable tensiometers for monitoring moisture potential in soil. As can be seen in FIG. 2 of the ""947, the tensiometer has a transducer 30 mounted externally to, and in fluid communication with, a fluid chamber 26. Because the transducer of ""947 is mounted externally, a disadvantage of the ""947 patent is that the pressure measured by the transducer is subject to changes in both the water level within the chamber as well as the changing air pressure of the headspace above the water level within the chamber. Also, the amount of water that can be contained within the chamber of the ""947 invention is limited by the length of the column of water in the water chamber thereby restricting the operating period of the tensiometer. The invention disclosed ""947 has the pressure sensor on the top of the instrument to measure the soil water potential in the adjacent sediment. The pressure sensor is located on the top of the instrument for ease of connection and to reduce the overall size (diameter) since it is vertically oriented to be placed in a borehole.
It is an object of the present invention to provide a tensiometer that automatically corrects the pressure measurement so that decreasing water levels within the tensiometer do not affect the recorded pressure measurements.
It is another object of the present invention to provide a tensiometer capable of longer operation periods as a result of its increased water volume capacity.
It is still a further object of the present invention to provide a tensiometer that will record pressure changes relative to atmospheric pressure and is independent of changes in water level within the tensiometer.
To achieve the foregoing and other objects, the present invention provides a self-compensating tensiometer that measures the total head in the system (i.e., the pressure head and the water level elevation head). In one preferred embodiment the self-compensating tensiometer comprises a body having a porous cap at one end and a chamber filled with liquid within the body. A pressure transducer housing contains a transducer sensor, the housing being immersed within the liquid. A first side of the transducer housing being in fluid communication with the liquid contained within the tensiometer housing. A second side of the transducer housing is vented through a reference port and vent tube to the atmosphere (either at the earth surface or the ambient atmosphere within the bore hole). In another embodiment of the invention, the transducer housing, reference port and portion of the reference vent tube are embedded in substantially solidified material (for example epoxy) to stabilize these components within the tensiometer. In this embodiment, a portion of the transducer housing may extend beyond the stabilizing material to provide fluid communication with the liquid within the tensiometer, or alternatively fluid communication means could be provided through an opening provided in the stabilizing material. A method for utilizing the self-compensating tensiometer is also claimed.